Add abstraction for "wheel" input source actions

[lread]

Problem/Opportunity

The W3C WebDriver spec describes a "wheel" input source for actions.
Etaoin has abstractions for "key" and "pointer" input sources, but not "wheel".

Proposed Solution

Study the spec, at the very least, describe the "wheel" input source in the docs, and if it makes sense, provide abstractions.

Additional context

Etaoin already has scroll* functions which are implemented by sending JavaScript to the browser.
This seems to work fine. Adding wheel actions doesn't necessarily mean deleting these scroll functions nor their implementation.

Current browser/driver support for actions using wheel.

Action

I can take a peek sometime.
It would probably also be nice to add a simpler example of using actions in the user guide.
It dives into a complex example straight away and implies actions are complex.